The speed of Metal-Oxide-Semiconductor (MOS) transistors is closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high.
Compound semiconductor materials of group III and group V elements (referred to as III-V compound semiconductors hereinafter) are good candidates for forming transistors due to their high electron mobility. Therefore, III-V based transistors have been explored. III-V compound semiconductor films, however, need to be grown on other substrates because it is difficult to obtain bulk III-V crystals. The growth of III-V compound semiconductor films on dissimilar substrates faces difficulties because these substrates have lattice constants and thermal expansion coefficients different than that of the III-V compound semiconductors. Various methods have been used to form high quality III-V compound semiconductors. For example, III-V compound semiconductors were grown from trenches between shallow trench isolation regions to reduce the number of threading dislocations.
The formation of III-V compound semiconductors from trenches typically includes an epitaxy growth, followed by a Chemical Mechanical Polish (CMP) to remove excess III-V compound semiconductors over the shallow trench isolation regions. The profile of the resulting structure, however, is difficult to control. Step heights, which are the height differences between the top surfaces of shallow trench isolation regions and the top ends of the III-V compound semiconductors, are high. Furthermore, the III-V compound semiconductors may have a significant dishing effect.